A hydraulic control system of an automatic power transmission generally includes shift valves by means of which upshifts and downshifts are to be automatically effected between different forward drive gear ratios. Each of the shift valves includes a valve spool to be acted upon by a throttle pressure variable with the load on the engine or by a line pressure variable with the throttle pressure and further by a governor pressure which is variable with vehicle speed. The shift points at which upshifts and downshifts between the gear ratios allocated to a shift valve are thus determined in relation to the vehicle speed and the load on the engine.
A shift valve incorporated into the hydraulic control system of an automatic power transmission is further designed so that the valve spool forming part of the shift valve is further responsive to a kickdown fluid pressure under conditions in which the accelerator pedal of the vehicle is depressed all the way down. The kickdown pressure thus acting on the valve spool of the shift valve urges the valve to make a downshift between the gear ratios for which the shift valve is responsible. The shift valve thus urged to make a downshift is allowed to be conditioned to make an upshift when the vehicle speed in icreased and as a consequence the governor pressure acting on the shift valve is increased to certain values which vary with the throttle pressure.
In a conventional shift valve, the kickdown pressure is caused to act on that pressure acting area of the shift valve which is normally acted upon by a throttle pressure or line pressure so that the shift points to be achieved during kickdown condition can not be determined independently of the shift points to be achieved during part-throttle conditions of the engine. For this reason, either the shift points for the kickdown condition or the shift points for the part-throttle conditions of the engine can not be made optimum without sacrificing the others.
Furthermore, a conventional shift valve is constructed and arranged in such a manner that the throttle pressure or line pressure to act upon the above mentioned pressure acting area of the valve spool is cut off from the pressure acting area once the valve spool is moved into the position producing the higher one of the gear ratios allocated to the shift valve for enabling the valve spool to act differently under upshifting and downshifting conditions and thereby providing different shift patterns for the upshifting and downshifting. In this instance, however, the shift pattern to govern the downshifting schedules is dictated solely by the throttle pressure or line pressure and, for this reason, either the shift pattern for the upshifting or the shift pattern for the downshifting must be sacrified if one of the two is to be selected as desired.
The present invention contemplates provision of a novel shift valve including two valve spools having respective pressure acting areas to be acted upon by the throttle pressure or line pressure and the kickdown pressure and arranged so that the kickdown pressure is applied to different pressure acting areas under kickdown and part-throttle conditions. One of the valve spools is used as a switching valve element and the other valve spool is used as a control valve element. Between these two valve spools is developed a throttle pressure or line pressure passed through a downshift valve provided in combination with the shift valve so that the shift valve is forcibly caused to make a downshift without respect to the downshift characteristics of the shift valve per se. Thus, the upshift pattern for the part throttle condition of the engine is dictated by the shift valve while the downshift pattern for the part-throttle condition of the engine is provided by the downshift valve so that the upshift and downshift patterns for the part-throttle conditions and the upshift and downshift patterns for the kickdown conditions can be predetermined independently of one another.
In accordance with the present invention, furthermore, a shift composed of two spools valve as above described are further arranged so that the fluid pressure contributive to the selection of a manual forward drive range of the transmission system is applied to the adjacent end faces of the valve spools so that the valve spool acting as the switching valve element is held in the downshift position when the manual forward drive range position is selected by the transmission gear shift valve. With this arrangement, the second or first gear position lock valve usually incorporated into a known hydraulic control system of an automatic power transmission system can be dispensed with.